doc: shellAfterCompleted should use `output`, not `stdout` [skip ci]

config.yaml

@@ -54,7 +54,7 @@ actions:
   # runs for too long.
   - title: Run backup script
     shell: /opt/backupScript.sh
-    shellAfterCompleted: "apprise -t 'Notification: Backup script completed' -b 'The backup script completed with code {{ exitCode}}. The log is: \n {{ stdout }} '"
+    shellAfterCompleted: "apprise -t 'Notification: Backup script completed' -b 'The backup script completed with code {{ exitCode}}. The log is: \n {{ output }} '"
     maxConcurrent: 1
     timeout: 10
     icon: backup